14:15 〜 14:30
[PPS01-15] EMIC Waves at Ganymede Detected by Juno’s Magnetic Field Observation
キーワード：木星、ガニメデ、EMIC波、Juno
Ganymede is embedded within the sub-Alfvénic plasma flow of Jupiter’s magnetosphere. Both Io- and Europa-genic ions are transported outward and dominant in the magnetospheric plasma sheet at the orbit of Ganymede (~15 Jupiter radii; 1 RJ = 71,492 km). Ganymede itself adds hydrogen and oxygen ions to the magnetosphere as well. Pickup ions from Ganymede’s atmosphere were detected by the Juno spacecraft during the close flyby at Ganymede on 7 June 2021 (Valek et al., 2022; Allegrini et al., 2022). Juno was connected with Ganymede’s intrinsic magnetic field in the plasma downstream. Direct measurements of ions by the Jovian Auroral Distributions Experiment (JADE) detected pickup-ion components with 10’s–100’s of eV, which are lower than the Jovian plasma (> 1 keV). Newly ionized pickup ions in the corotating magnetic field are accelerated perpendicularly to the magnetic field, so that they initially form a highly unstable, ring-shaped velocity distribution perpendicular to the magnetic field, which generates electromagnetic ion cyclotron (EMIC) waves near and below individual ion cyclotron frequency (Huddleston et al., 1998). Previously, EMIC waves associated with pickup ions were detected in the plasma downstream of Io by the Galileo spacecraft but have yet to be detected at Ganymede.
In this study, we investigate Juno’s magnetic field data obtained in the Ganymede flyby and examine the low-frequency wave characteristics. The Magnetic Field Investigation (MAG; Connerney et al., 2017) onboard Juno measured the magnetic field at a rate of ~64 Hz during the flyby. We use wavelet transform to examine low frequency waves in 10-2–101 Hz. The wavelet scalogram from the observed magnetic field data shows enhancement of wave amplitude near the local cyclotron frequency (fi) of ions with mass-per-charge of i = 1, 2, 16, and 32 AMU/q inside the Ganymede magnetosphere. We then take the singular value decomposition (SVD) method (Santolík et al., 2003) and calculate wave planarity, polarization, and wave normal angle to interpret the wave characteristics. Based on the wave analysis with the SVD method, we find that the EMIC waves associated with 32-AMU/q ions exhibited left-handed polarization in the Ganymede magnetosphere. We also provide the hodogram analysis near f16because the rotational direction was not interpreted accurately from the SVD-derived polarization values due to the low planarity near f16. The hodogram shows clear right-handed polarization just below f16, which indicates the presence of the R-mode EMIC waves associated with 16-AMU/q ions. Combined with the results from the simultaneous ion measurements, we conclude that the detected EMIC waves are associated with O+ and O2+ pickup ions originally from Ganymede. We also suggest that excitation process of the EMIC waves associated with pickup ions is shared in the plasma downstream of Io and Ganymede. This study provides an example of thorough examination of low frequency waves in a compact magnetosphere and imprints a framework that could be extended to similar wave analysis on other compact magnetospheres such as Mercury’s.
In this study, we investigate Juno’s magnetic field data obtained in the Ganymede flyby and examine the low-frequency wave characteristics. The Magnetic Field Investigation (MAG; Connerney et al., 2017) onboard Juno measured the magnetic field at a rate of ~64 Hz during the flyby. We use wavelet transform to examine low frequency waves in 10-2–101 Hz. The wavelet scalogram from the observed magnetic field data shows enhancement of wave amplitude near the local cyclotron frequency (fi) of ions with mass-per-charge of i = 1, 2, 16, and 32 AMU/q inside the Ganymede magnetosphere. We then take the singular value decomposition (SVD) method (Santolík et al., 2003) and calculate wave planarity, polarization, and wave normal angle to interpret the wave characteristics. Based on the wave analysis with the SVD method, we find that the EMIC waves associated with 32-AMU/q ions exhibited left-handed polarization in the Ganymede magnetosphere. We also provide the hodogram analysis near f16because the rotational direction was not interpreted accurately from the SVD-derived polarization values due to the low planarity near f16. The hodogram shows clear right-handed polarization just below f16, which indicates the presence of the R-mode EMIC waves associated with 16-AMU/q ions. Combined with the results from the simultaneous ion measurements, we conclude that the detected EMIC waves are associated with O+ and O2+ pickup ions originally from Ganymede. We also suggest that excitation process of the EMIC waves associated with pickup ions is shared in the plasma downstream of Io and Ganymede. This study provides an example of thorough examination of low frequency waves in a compact magnetosphere and imprints a framework that could be extended to similar wave analysis on other compact magnetospheres such as Mercury’s.